The Great Christ Comet (21 page)

Essentially, meteors are bright streaks in the night sky that occur when meteoroids crash into Earth's upper atmosphere. The meteoroids begin to heat up when encountering resistance at 130–150 km altitude. By the time they have reached 100 km they are bright enough to be visible on Earth as “shooting stars.” By 75 km, most have disappeared. Larger meteoroids, however, are able to penetrate further through the atmosphere and tend to produce brighter, bigger meteors. Especially bright meteors are called fireballs; their brightness will be equal to or greater than that of the planets Jupiter or Venus.
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When the trails of a number of meteors seem to point back to one particular point in the sky, called a radiant, they are regarded as constituting a “meteor shower.” Whenever astronomers judge that more than 1,000 meteors would have been observable per hour in a dark sky had the radiant been at the zenith (the imaginary point immediately above an observer's head), they classify the phenomenon as a “meteor storm.”

During the great meteor storm of 1833, hundreds of thousands of meteors seemed to radiate from the head of Leo the lion, causing many observers in North America to imagine that the stars were falling from the sky, that the heavens were on fire, and that the world was coming to an end. This and other Leonid meteor storms before and since then were due to a dense section of meteoroids along the meteoroid stream parented by the Halley-type comet Tempel-Tuttle. Meteor storms radiating from the constellation Andromeda in 1872 and 1885 were obviously linked to the splitting and disintegration of the Jupiter-family Comet Biela over the previous few decades. Meteor showers and storms are usually related to Jupiter-family or Halley-type comets. However, some meteor outbursts have been identified with long-period comets with orbital periods of less than 10,000 years that come within 0.1 AU of Earth.
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Most meteors, however, are “sporadic,” that is, they are not associated with any particular shower. Many undoubtedly did in the distant past belong to clearly defined meteor showers, but it has now been so long since they were last replenished with fresh debris that the streams are too diffuse to be identified as showers.

Great Comets

Some comets set themselves apart from the majority by virtue of their sheer magnificence—their brightness and largeness and/or length seen against the backdrop of a dark sky. Such comets are classified as “great comets.”

There is widespread agreement regarding the attributes that render a comet “great.”
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First, a comet may make a close pass by the Sun. Chief among those attaining to greatness primarily because they came close to the Sun are the Kreutz Sungrazers. For example, the sungrazing Great March Comet of 1843 (see figs. 5.27–28) came to within 0.005 AU of the Sun (about 130,000 km from the Sun's surface),
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traveling so fast (560 km a second) that it made its way three-fourths of the way around the Sun in less than 12 hours,
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and attaining to -7 to -10 magnitude.

Second, a comet may make a close approach to Earth. The closer a comet comes to our planet, the brighter it seems to human observers—even normally dim objects can become remarkably bright when they make close passes by Earth. Among the great comets notable for coming near to Earth is the Great Comet of 1861, which came as close as 0.13 AU to Earth, and Comet Hyakutake, which came to within 0.1 AU of Earth on March 25, 1996.

Third, a comet may develop a large, bright coma. Since large nuclei tend to have more volatiles to react to the Sun and hence become more active, they usually form bigger comas. Among those considered great primarily because of the large size of their comas are the Great Comet of 1811 (see figs. 5.29–31) and Hale-Bopp in 1996–1997, neither of which came close to the Sun or Earth.

Fourth, a comet may sport an eye-catchingly long tail. A great comet will generally have a tail that is 10 degrees or more in length. The Great Comet of 1618 (C/1618 W1) and Tebbutt's Comet of 1861 are two striking examples of comets rendered great because of their impressively long tails, 104 and 120 degrees respectively.

Fifth, a comet should offer the general public in the more populated northern hemisphere good viewing opportunities, preferably in a dark sky (at least 16–20 degrees from the Sun) in the hours after sunset, and should capture the public's attention. In addition, the comet should be clearly visible to the naked eye for a significant period of time.

Almost all great cometary apparitions are associated with long-period comets. The only short-period comet whose apparitions may be considered “great” is Halley's Comet.

Ancient Cometary Records

While the places of the planets and Sun and Moon could be established by the ancients by means of calculation, the unpredictability of comets meant that they had to be observed and careful records of each stage of their apparition kept. We are fortunate that the Bab­ylo­nians and Far Easterners made astronomical observations on a regular basis and over a long period. Their reason for doing so was largely their belief in astrology.
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Babylonian Records

The Bab­ylo­nians kept astronomical diaries, namely detailed records of their celestial observations, for many centuries, from the middle of the eighth century BC right up to the first century AD. These records refer to a comet as a
sallammu
or
sallummu
.
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Unfortunately, no Bab­ylo­nian records of comets survive from the fifty years either side of the birth of Jesus.
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Only nine Bab­ylo­nian comet records have survived, and they only in fragmentary form, mentioning comets in 234, 210, 164, 163, 157, 138, 120, 110, and 87 BC.
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The 164 BC and 87 BC comets are believed to be apparitions of Halley's Comet.
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It is worth briefly overviewing some of these Bab­ylo­nian cometary records, preserved on surviving fragments of cuneiform texts, to get a sense of what the Bab­ylo­nian astronomers tended to take note of regarding comets. Concerning the 234 BC comet, they noted that it was first observed in the east in the last watch of the night sometime in January/February. With respect to the 210 BC comet, they recorded that it first appeared along the ecliptic in Scorpius, with its tail pointing eastward, in June/July. Regarding the 164 BC comet (Halley's), the Bab­ylonians took note of the fact that the comet had appeared in the area of the Pleiades and Taurus and then, with its tail 7½ degrees long and oriented northward, moved to about 2½ degrees from Jupiter within Sagittarius. One cuneiform fragment noted that the 163 BC comet had a southward-oriented tail and was located 1½ degrees above the star
α
(Alpha) Coronae Borealis on September 5; another, more damaged fragment compares the comet's location to that of the same star and identifies the time as the first watch of the night. Only the date of the 157 BC comet, in October/November, has survived. With reference to the 138 BC comet, the Bab­ylonians noted where and when the comet became stationary, and the zodiacal location (Libra) and date (May 28) of the comet's heliacal setting (its final visible setting in the evening in the run-up to a period when it is invisible due to its closeness to the Sun) in the west. With respect to the 120 BC comet, fragments of several records have survived, detailing: (1) the celestial location of the comet on May 18; (2) where and when it became stationary (May 20); (3) that its tail pointed southward on June 16; and (4) that the comet was seen on July 13 at the beginning of the night and that it had previously appeared in Aries in the east on day 29 of an earlier Bab­ylo­nian month (probably equivalent to May 18). Concerning the comet of 110 BC, the Bab­ylonians noted, on November 23, the part of the sky—the east—where the comet was and indeed its precise celestial location and the orientation of the tail—pointing westward. A later record recalled that initial observation and mentioned that the comet had subsequently migrated into the northern sky (or perhaps simply north of some celestial entity). Finally, with regard to the 87 BC (Halley's) comet, the Bab­ylonians recorded that it was seen in the first part of some night in July/August and commented on the rate of its motion through the sky (“day beyond day one cubit”) and the northwestern direction of the 10-degree tail.
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